In the past, oil shale deposits were mined and brought to the surface for further processing of the various components and constituents. This process was expensive, time-consuming, and dangerous. If the oil shale deposits were mined by open pit, their removal was time-consuming and expensive. Additional ecological problems render both these methods of extraction undesirable today.
A somewhat more dangerous approach involves underground tunneling into the shale oil deposits in a predetermined pattern for the purpose of blasting and rubblizing the oil shale deposit. After the deposit is rubblized, a flame front is instituted which causes an in-situ retorting of the hydrocarbon values in the shale. This process has met with varying success primarily because of difficulty of obtaining uniform rubble in the shale deposit with the attending problems of maintaining a reasonably uniform flame front and plastic flow of the rock material. If the rubble is not reasonably uniform, a substantially uniform flame front is not maintained and the retort flames are quenched by the retorting products or by-pass burning occurs. The plastic flow problems are particularly severe in the deposits richer in kerogen.
Various forms of pressure swings have been used in the past to improve recovery from oil fields. In one process, a down-holed gas/oxygen gun propagates shock waves through an oil field to crack the underground formation, thereby releasing additional pockets of oil. In another process, steam is cycled (huff and puff) so as to recover viscous oil from sand and gravel. Neither of these processes are suitable for the present invention. Shock waves play no part in the present process, and it is often desired to avoid further cracking rather than to cause it. With respect to the cycling steam process, it is applied only until the heated subsurface area of two adjacent wells come into contact and then it is replaced by continuous steam pressure drive. Moreover, the formations wherein "huff and puff" has been applied are essentially a mixture of heavy oil, sand, and gravel. They have neither the prominant horizontal layered structure nor the blind cracks of the oil shale deposits.